1. Field of the Invention
The present invention relates to a speech analyzing and synthesizing apparatus in which a speech signal is analyzed and coded for transmission and preserved by shortening the average code length and, also to a speech recording apparatus in which a soundless portion of a speech signal to be recorded is compressed when the speech signal is analyzed and coded.
2. Description of the Prior Art
Generally, a large volume of information content is necessary for expressing speech signals. However, since the speech signals have a considerable redundancy, they can be transmitted with high efficiency and recorded with small capacity if they are compressed.
Coding of the speech signals is divided roughly into two methods, that is, waveform coding, and coding in the frequency domain. The waveform coding outputs a fixed length of coded bit (information content) for example, per every sample, while the coding in the frequency domain outputs a fixed length of coded bits for example, per frame. However, it is better to output more bits when the waveform changes severely, bringing about difficulties for prediction, and less bits when the waveform changes slightly, with easy prediction, that is, variable code-length coding is better than waveform coding whereby a fixed length of bit are outputted per every sample, because the average code length can be shortened in the variable code-length coding, which is suitable for recording with small capacity. This can is also true with respect to domain coding.
Conventionally, in the case where there is non-uniformity in the amplitude distribution of a speech waveform, that is, an inclination in the probability distribution of a code indicating each amplitude, the speech signals are coded into variable length codes in which the information (bit length) of each code is changed depending on the appearance frequencies of quantization results, so that the average code length can be shortened. This method is called an entropy coding, such as Huffman code. In the conventional variable code-length coding such as the above-mentioned entropy coding, however, since the coding method is based on statistical characteristics such as generation probability of each information segment in an inputted speech signal, etc., the statistic characteristics must should be studied in advance so as to form a variable length code that can render the entropy a minimum. Moreover, the bit length of a coded signal changes so greatly in the entropy coding that difficult countermeasures against transmission error must be taken.
Also, the speech signals include a soundless pause section in which no sound is generated during speech. Therefore, if the information in the soundless section is compressed, the speech signals can be transmitted with high efficiency, and the recording capacity of the memory can be reduced. Particularly, for a in the normal speaking speed, sounded section in which speech is actually generated covers approximately only 68% of all speaking time, and therefore the compression of the soundless sections can reduce a considerable volume of speech information. In the conventional speech recording and reproducing apparatus, a sound judging means determines once per frame whether the speech signal is a sounded signal or a soundless signal and then the soundless section is compressed. Therefore, when a the speech signal with a compressed soundless section is reproduced, the beginning or the ending of a word in the speech is disadvantageously omitted. Moreover, in the conventional apparatus, a weak short-time section in the inputted speech signal (for example, a double-consonant in Japanese "Gakko", etc. or an assimilated sound) is erroneously judged to be soundless and accordingly erroneously compressed. Therefore, in reproducing the inputted speech signal, the double-consonant portion is treated as a perfect soundless section (for example, "Ga-ko" in Japanese), bringing about a strange and odd feeling in hearing.